A vinyl chloride-based polymer is a polymer including 50% or more of vinyl chloride, cheap and easy to control the hardness thereof, and applicable in most processing instruments, and thus, has various application fields. In addition, since molded articles having excellent physical and chemical properties, for example, mechanical strength, weather resistance, chemical resistance, etc. may be provided, the vinyl chloride-based polymer may be widely used in various fields.
However, due to chemical structural defects generated during a polymerization reaction, dehydrochlorination from the vinyl chloride-based polymer by heat or ultraviolet rays applied during processing may occur, and discoloration of a resin may be induced or the physical properties thereof may be deteriorated.
In particular, in a vinyl chloride-based polymer, chemical defects generated during a polymerization reaction, that is, chemical structural defects such as allyl chloride and tertiary chlorine may be present in the vinyl chloride polymer. Due to such chemical structural defects, the bonding energy between carbon and chlorine in the vinyl chloride-based polymer is very low than the bonding energy between carbon and chlorine in a normal molecular structure, bonding between carbon and chlorine may be easily cleaved due to external radical transition during the processing of the vinyl chloride-based polymer, and hydrogen chloride separated from a molecular chain may accelerate a new side reaction due to an auto-catalyst reaction, thereby continuously generating hydrogen chloride. In addition, a double bond may be formed at the position where the hydrogen chloride is left, and with the accumulation of such double bonds, problems of the discoloration or decrease of physical properties of a resin may be generated. That is, a dehydrochloric acid (dehydrochlorination) reaction is generated from a vinyl chloride-based polymer or a molded article processed therefrom due to heat or ultraviolet rays, and as a result, defects of discoloration or decrease of physical properties of the vinyl chloride-based polymer itself may occur.
In order to improve such defects of the vinyl chloride-based polymer, an organometallic compound containing a metal such as Ba, Zn, Ca and Pb is mixed with the vinyl chloride-based polymer, so as to suppress the generation of radicals or ions produced during thermal decomposition of the vinyl chloride-based polymer and to control the thermal decomposition rate of a resin. Recently, a method of using a thermal stabilizer having various types such as metallic types or organic compound types has been introduced, but due to environmental problems and high price caused by using a heavy metal stabilizer, the use thereof has been largely restricted.
In addition, a method of blending a polymer having good heat resistance with a vinyl chloride-based polymer to compensate weak physical properties has been suggested, but the use thereof is not easy because of difficult processing due to low miscibility with the vinyl chloride-based polymer.
Accordingly, development of a technique which is capable of effectively improving the thermal stability of a vinyl chloride-based polymer is required.